As was stated by others, Southwest only flies 737s. Even when they bought out AirTran, they only kept their 737s, and sold off everything else. The largest version they currently use is the 737-800, but that's longer but not wider than the other 737s in their fleet.
Also - SNA is notorious for both having one of the shortest runways anywhere that's used for large commercial jets at about 5700 ft. That's coupled with their noise abatement requirements that basically force pilots to drag race, climb near maximum thrust at a steep angle, and then cut their engines severely during the climb to altitude. It's supposed to reduce the amount of noise that reaches the residential areas, but some call it a "white knuckle ride".
As for size of aircraft, there's never been any scheduled service out of SNA of anything larger than a 757. The runway is too short for a widebody plane like a 747/767/777 or Airbus A330. Coast to coast routes are served by jumbos, but not for a three hour flight. Most planes that size are used for transoceanic flights.
Southwest only seems to have two nonstop flights (#2846/#1233 weekdays, #276/#1233) a day DAL-SNA.